water Article Digestate Liquid Fraction Treatment with Filters Filled with Recovery Materials Ilaria Piccoli 1 , Giuseppe Virga 2, Carmelo Maucieri 1,* and Maurizio Borin 1 1 Department of Agronomy, Food, Natural Resources, Animals and Environment (DAFNAE), University of Padova, Viale dell’Università, 16, 35020 Legnaro, Italy; [email protected] (I.P.); [email protected] (M.B.) 2 Consorzio di Ricerca per lo Sviluppo di Sistemi Innovativi, via Libertà, 203, 90133 Palermo, Italy; [email protected] * Correspondence: [email protected] Abstract: Constructed wetlands (CWs) represent a green technology for digestate liquid fraction (DLF) treatment. However, previous research has warned about their performance when treating wastewater with high suspended solid and organic loads. In addition, the high NH4-N concentration typical of this wastewater can compromise vegetation establishment and activity. In view of this, a digestate pretreatment is needed. This study aimed to test the performance of filters filled with recovery materials, such as brick and refractory material, for DLF pretreatment. The effect on DLF physical (electrical conductivity, pH, dissolved oxygen, and temperature) and chemical (total nitrogen, ammonia–nitrogen, nitrate–nitrogen, total phosphorus, soluble phosphorus, and chemical oxygen demand) characteristics was monitored during eight weekly cycles. The effect of filtration on total nitrogen and ammonia–nitrogen removal began after about one month of loading, suggesting that an activation period is necessary for bacteria. For effective N removal, the presence of multiple digestate recirculations per day through the filters appears mandatory to guarantee the alternation of nitrification and denitrification conditions. For P removal, filling material particle size appeared to be more important than its composition. Unclear performances were observed considering chemical oxygen demand. Further studies on filling media and microbial community interactions, and the Citation: Piccoli, I.; Virga, G.; long-term efficiency of filters, are desirable. Maucieri, C.; Borin, M. Digestate Liquid Fraction Treatment with Keywords: digestate; aerobic filters; nutrients removal; wetland pretreatment Filters Filled with Recovery Materials. Water 2021, 13, 21. https://doi.org/ 10.3390/w13010021 1. Introduction Received: 2 October 2020 Accepted: 18 December 2020 Electricity production from biogas plants in the European Union (EU) has grown Published: 24 December 2020 rapidly during the past 25 years, accounting for half of the global biogas production (ca. 650 PJ) in 2015 [1]. The biomass of digestate, the by-product of anaerobic digestion, is Publisher’s Note: MDPI stays neu- quantitatively similar to the inlet raw materials. Indeed, during anaerobic digestion, a tral with regard to jurisdictional clai- fraction of organic carbon is transformed into methane (CH4) and carbon dioxide (CO2), ms in published maps and institutio- whereas for most of the other elements, particularly nitrogen (N) and phosphorus (P), there nal affiliations. is no material loss but rather a form change [2,3]. The N recovery in the digestate is close to 100%, whereas P, calcium (Ca), magnesium (Mg), and manganese (Mn) are partially crystallized in substituted phosphate form, which can be used as P fertilizer [4]. Generally, + Copyright: © 2020 by the authors. Li- digestates are characterized by a higher ammonium (NH4 )-to-total-N (TN) ratio; stable censee MDPI, Basel, Switzerland. organic matter (OM) and pH; and lower easily degradable OM, total and organic carbon (C) This article is an open access article content, biological oxygen (O2) demand (BOD5), C-to-N ratio, and viscosity compared with distributed under the terms and con- undigested raw materials [5]. Nonetheless, digestate composition is a function of incoming + ditions of the Creative Commons At- biomass [6,7]. For example, the NH4 -N content is directly related to feedstock total N + tribution (CC BY) license (https:// content. Indeed, feedstock-derived digestates are characterized by a high NH4 -N-to-TN creativecommons.org/licenses/by/ ratio and a low C-to-N ratio [8,9] due to the high degradability of the raw materials (e.g., 4.0/). Water 2021, 13, 21. https://doi.org/10.3390/w13010021 https://www.mdpi.com/journal/water Water 2021, 13, 21 2 of 12 cereal grains). In contrast, cattle manures or fibrous feedstocks with low N content (e.g., + silage maize) lead to a low NH4 -N-to-TN ratio digestate [10]. Digestate is commonly recycled as fertilizer [11], but if its total N content is expected to be higher than the EU Nitrates Directive limits per spreading unit surface, it has to be treated for N reduction [12,13] before agriculture use. Different types of digestate treatment are reported in the literature, from liquid/solid separation to membrane filtration [14,15]. However, these are expensive to realize and maintain. Separation creates two outputs— a solid and a liquid fraction—that need to be stored and handled separately [16]. The former represents about 15% of the raw digestate and is concentrated in organic matter, as organic N and P, and is a valuable and easily manageable soil amendment [17]. The liquid fraction, which represents about 85% of the raw digestate, is rich in nitrogen (about 0.5% of fresh weight), mainly as ammonium (about 70%) [18]. Considering the European Nitrates Directive limits (from 170 to 340 kg N ha−1 in relation to vulnerability of spreading area) and taking into account the liquid fraction characteristics, its management is more difficult than that of the solid fraction and requires a large land area for spreading. In addition, its management implies the costs of transportation and distribution. For these reasons, the reduction in the N content in this fraction is of crucial importance to reduce the required land area and related costs of distribution. Constructed wetlands (CWs) represent a green technology for digestate treatment [19,20]. They are engineered systems that exploit natural processes to remove organic compounds and nutrients from wastewater [21–24] and provide effective, low-cost, low-maintenance, and environmentally beneficial alternatives to traditional systems [25,26]. Despite the numerous reported benefits, previous research has warned about the long-term function of CWs as a form of treatment for wastewater characterized by high organic and suspended solid loads [27–29] as + digestate [19,30] due to substrate clogging. Additionally, the high NH4 -N and salinity content may compromise the plant survival in CWs [26]. In several papers, the digestate liquid fraction (DLF) was diluted (from 1:2 to 1:20 DLF:water) before CW treatment, and in some cases it was additionally subjected to a residence time in a sediment tank [13,19,20,31] with the aim to reduce + the suspended solids and NH4 -N concentration. However, dilution may not be considered a suitable solution in practical conditions because it implies higher volumes to store and manage. For this reason, a digestate pretreatment able to reduce the pollutant load before CW treatment is needed. Different pretreatment technologies are available, such as ultrafiltration, reverse osmosis, and stripping, that also allow nutrient recovery, but these are relatively complex and expensive [32] from both economic and environmental perspectives. Therefore, interest in exploring more environmentally friendly pretreatment solutions remains. A possible low-cost pretreatment system, already used for other effluents, is aerobic filtration [33,34]. Although sand-filled filters are the most commonly used approach [35–39], several investigations of on-farm and/or low-cost filtering media have been undertaken, such as wood chips, wheat straw [40], oyster shells [41], semisoft plastic media [34], rockwool [42], and materials from the recirculation of subproducts of different industrial processes (e.g., laterite [43], zeolite, and zeolitite [44,45]). In this context, the individuation of low-cost and easily available filling materials is a target that can combine the needs of developing filters for DLF pretreatment and fostering the circular economy of farms [46,47]. However, to our knowledge, no studies have been conducted on the use of recovery materials for filtration before DLF CW treatment. Therefore, the aim of this study was to test the effects of recovery materials used as filtering media, such as fragmented bricks and refractories, on the main DLF chemical– physical characteristics and to evaluate whether the resulting wastewater presents adequate characteristics to be treated with CWs. 2. Materials and Methods 2.1. Filter Structure and Composition The experiment took place indoors from March to May 2013 and involved four alu- minum filters composed of two modules each (0.48 m height, 0.48 m diameter, 0.08 m3 volume) (Figure1). The upper module was a real filter containing the filling medium held Water 2021, 13, x FOR PEER REVIEW 3 of 13 Water 2021, 13, 21 3 of 12 The experiment took place indoors from March to May 2013 and involved four alu- minum filters composed of two modules each (0.48 m height, 0.48 m diameter, 0.08 m3 volume) (Figure 1). The upper module was a real filter containing the filling medium held by a steel plate mesh and covered with a plastic cap to prevent evaporation. The lower by a steel plate mesh and covered with a plastic cap to prevent evaporation. The lower module wasmodule a tank inwas which a tank the in which DLF was the DLF stored was after stored filtration. after filtration. Upper Upper and lower and lower modules modules were connectedwere throughconnected a through vacuum a pump,vacuum which pump, was which controlled was controlled by a timerby a timer set to set a to 3 ha 3 h timespan, allowingtimespan, eight allowing filtration eight cyclesfiltration per cycles day. per day. Figure 1. Description of filter structure: (a) upper (UM) and lower (LM) filter modules; (b) mod- Figure 1. Description of filter structure: (a) upper (UM) and lower (LM) filter modules; (b) modules’ ules’ internal structure with dimensions expressed in mm; (c) vacuum pump; (d) plastic cap; (e) internal structuretimer.